HIV incidence and mortality decreased globally yet increased markedly in Eastern Europe and Central Asia (EECA) where the HIV epidemic remains volatile and fueled primarily by people who inject drugs (PWIDs), primarily of opioids. Harsh penalties for drug use have concentrated PWID in prisons. Our recent findings in Kyrgyzstan (KYR) confirm HIV high rates of opioid injectors (30.4%) and HIV (10.2%), with 85% of PWID having injected within prison. KYR presents a unique and timely opportunity to research and control HIV transmission in prisons as one it is one of only 7 countries worldwide to offer prison-based methadone maintenance therapy (MMT). New mathematical modeling suggests within-prison MMT scale-up with post-release retention is the most effective HIV prevention strategy for incarcerated PWID in PWID-driven epidemics. Scaling-up MMT in prison, however, may be influenced by individual and environmental factors, but the HIV ?risk environment? framework for prisons has yet to be fully constructed nor applied empirically to PWID who transition from prisons to communities where the risk environment changes. Thus, researching the dynamics of prison-based MMT delivery in KYR will inform the implementation and expansion of MMT in prisons. This study builds on our research on HIV ?risk environments? by applying this framework for the first time, to explore how the prison environment amplifies HIV risk within prison and post-release. Structural approaches, which look beyond individual behaviors to address environmental risk, are integral to reducing HIV risk. Our recent prison research in Ukraine, Moldova, and Indonesia, suggests high-risk drug use is situated in an environment where a robust drug economy and social hierarchy co-exists and competes with MMT. In the absence of MMT retention in the community, the high-risk environment post-release differs from prison but also contributes to poor health outcomes. To address this transition, we propose to longitudinally explore the sparsely researched `enabling/disabling environment' potential of prison-based MMT during incarceration and during the tumultuous post-release period. This timely application is planned coincident with MATLINK (R01 DA029910) activities, a 6-country longitudinal implementation science study that uses motivational interviewing to expand MMT within EECA prisons by targeting the ?individual?. The proposed R21 research adds value by generating qualitative data with MATLINK participants and prison staff, adding study participants who do not receive motivational enhancement to start MMT, uses qualitative data to supplement structured surveys and then triangulates findings to better understand MATLINK survey data. The proposed mixed methods analysis is ideally suited to assess ?how? and ?why? would-be MMT patients and personnel engage with MMT and HIV risk in the transitional context of both the pre-release prison and post-release community environments. Findings will provide new insights into how to optimize biobehavioral and/or structural HIV prevention strategies in prisons, and optimize MMT scale-up and retention and reduce WPDI risks. Our two related specific aims are to: 1) describe the individual- environment interactions that shape within-prison and post-release drug-related HIV risk practices and health expectations for HIV+ and HIV- PWID; and 2) evaluate the how within-prison environment risk and other factors influence engagement with MMT both within prison and after release for HIV+ and HIV- PWID. Our research team brings together international researchers with expertise in qualitative and mixed-methods, implementation science, prisons, addiction treatment (MMT), and HIV prevention, and KYR collaborating sites with demonstrated successes working in the political and legal context in EECA countries and prisons. !